Answer:
Because Gram negative bacteria have an extra protective layer of lipopolysaccharide surrounding their cell wall!
Explanation:
Mechanism of action of penicillin is:
- The cell wall of bacteria is made of a macro molecule known as peptidoglycan.
- The cell wall is made by cross-linking of peptidoglycan with the help of an enzyme called D-D Transpeptidase (or the penicillin binding protein).
- The penicillin binds to this enzyme and inhibit its synthesis, and bacteria needs that enzyme to replicate thus stopping the cell cycle
Effect on gram negative :
As you can see in the image, the gram negative bacteria has an extra layer of protection on its cell wall by lipopolysaccharide. Penicillin can't reach the cell wall to bind to the D-D Transpeptidase, so it is more effective on gram positive bacteria than gram negative bacteria.
